The present invention relates to an alkali-proof cast aluminum product having a wear-resistant surface layer and a process for producing said product.
Today, cast aluminum products are extensively used in various fields of the industry. For example, they are utilized as a doorsill having the shape of FIG. 1 in the field of rolling stocks, as a step material for staircases, as a floor material, and so on. Particularly, the above-mentioned doorsill made of cast aluminum product is desirable from the viewpoint of reducing the weight of rolling stocks which is necessary for saving the driving energy and decreasing the load laid upon rails and bridges. Further, a surface structure exhibiting an antislip function is sometimes required of this type of cast aluminum product from the viewpoint of ensuring the safety of passengers getting on and off buses, trains and the like. For these purposes, it has been proposed to improve the wear-resistance and antislip function of the surface of base material by cast-embedding a number of wear-resistant granules of SiC, Al.sub.2 O.sub.3, corundum, alundum or the like so that the granules partially protrude above the surface.
Now, the surface of step decks and similar articles must be cleaned frequently, because it is soiled in a short period of time by many persons walking thereon. A cleanser used for cast aluminum product must be weaker in alkalinity than a cleanser for iron or steel, because aluminum is inferior to iron and steel in alkali resistance.
However, it is the actual state of the art at the present stage that, when cast aluminum products are used in combination with iron or steel products, nevertheless, a strongly alkaline cleanser suitable for iron and steel products is directly applied to aluminum products. Such a cleanser is so strongly alkaline as to have a pH value of 11.5 to 13.7. If such a cleanser is directly applied to cast aluminum product, the surface of the aluminum is slightly lost by dissolution (alkali corrosion), due to which the cast aluminum product having the above-mentioned surface structure decreases or loses the supporting power exercised on the wear-resistant granules so that the granules are lost from the surface and the wear-resistance and anti-slip function of the product are markedly deteriorated. In addition, a part of the strongly alkaline cleanser penetrates into the open cavities on the surface of the cast product to corrode the inner part of the product, as a result of which the cast aluminum product decreases its strength.